Sagnac distributed sensor

ABSTRACT

A system based on the usage of a Sagnac interferometer operating in a dual sensing mode is described which allows the determination of the location and amplitude of a time varying disturbance on a single fiber.

This is a division of application Ser. No. 208,982, filed June 20, 1988now U.S. Pat. 4,898,468, issued Feb. 6, 1990.

BACKGROUND OF THE INVENTION

This invention relates generally to fiber optic detection systems basedon the Sagnac interferometer. In one mode of operation the Sagnacinterferometer disclosed by this invention responds to time varyingenvironmental disturbances in a manner similar to that described byRichard Cahill and Eric Udd in U.S. Pat. No. 4,375,680, "OpticalAcoustic Sensor" and by Eric Udd in "Fiber-Optic Acoustic Sensor Basedon the Sagnac Interferometer", Proceedings of SPIE, Vol. 425, p. 90,1983. There it was shown that the amplitude of the response to a timevarying disturbance depends upon its position in the Sagnac loop wherethe sensing occurs. If the disturbance occurs near the center of theloop, the response becomes vanishingly small, while for a disturbancenear the ends of the loop, the response approaches the maximum possibleamplitude. In the second mode of operation, the Sagnac interferometer isconfigured to have sensitivity that is constant over the length of thesensing loop. These configurations are described in U.S. patentapplication Ser. No. 917,390, applied for by Eric Udd, et al, Now U.S.Pat. No. 4,787,744 issued on 11/29/88, and assigned to the assignee ofthe instant application.

John Dakin, in a paper entitled "A Novel Distributed Optical FiberSensing System Enabling Location of a Disturbance in a Sagnac LoopInterferometer", Proceedings of the SPIE, v. 838, p 325 (1987),describes a combination of Mach-Zehnder and Sagnac interferometers wherealong a single fiber optic path the Mach-Zender interferometer hasdirect detection sensitivity while the Sagnac interferometer hasposition dependent sensitivity as noted above. By ratioing the positiondependent and position independent signals, the location and magnitudeof the disturbance may be determined. This latter invention is severelylimited by the contradictory requirements of the Mach-Zehnder and Sagnacinterferometers. For the Mach-Zehnder interferometer optimum performanceis achieved by utilizing a long coherence length light source with highfrequency stability. The performance of these light sources degraderapidly with light feedback into the source. The Sagnac interferometerhas optimum performance when a low coherence length light source isused, and its performance degrades rapidly as the coherence lengthincreases due to Rayleigh backscatter from the sensing loop. Thecontradictory requirements of these interferometers result in a lightsource which is a compromise resulting in substantial limitations in theperformance of one or both interferometers in the Dakin device severelylimiting performance. The situation is further aggravated by thecombination of Sagnac and Mach-Zehnder interferometer as described byDakin resulting in direct feedback of the signal light into the lightsource, which results in a worst case scenario for the light source thatwould optimize the performance of the Mach-Zehnder.

What is desired is a fiber optic sensor that is capable of sensing boththe location and magnitude of a disturbance along a single fiber withoutthe limitations and excess noise generated by mixing the highlyincompatible Sagnac and Mach-Zehnder interferometers.

SUMMARY OF THE INVENTION

The device and method of the present invention allows the magnitude andlocation of a time varying disturbance to be determined along a singlefiber by using Sagnac interferometer based configurations that operatein modes that allow position independent sensitivity along the entirefiber sensing loop in one mode and position dependent sensing in thesecond mode. These outputs are then processed to determine position andmagnitude of the disturbance. In one simplified embodiment, themagnitude and location of the disturbance is determined sequentially byswitching between the two modes of operation. In other more complex andefficient embodiments, both the magnitude and location of thedisturbance are simultaneously determined.

Omitting, for the purposes of this summary an explanation of the moretechnical performance elements of the system, the invention consists ofa light source which injects light into a fiber and beam conditioningelements before being split into counterpropagating light beams around aloop of optical fiber by a light beamsplitter. Within the sensing loopthere is an optical frequency shifting element. This may be operated inan "off" condition where both counterpropagating beams pass through itwithout being frequency shifted. In this mode of operation when a timevarying disturbance impinges on the fiber loop, the relative phase shiftgenerated between the counterpropagating light beams will depend on thelocation of the disturbance.

For example, if the disturbance is located near the center of thesensing loop, both counterpropagating light beams will arrive atessentially the same time and experience essentially the samedisturbance. The net result is that the relative phase shift between thecounterpropagating light beams will be small and when the two beamsrecombine at the beamsplitter the intensity change due to interferencebetween the two beams will be small. When the disturbance occurs at aposition that is substantially offset from the center of the fiber loop,the two beams arrive at significantly different times and thedisturbance has time to effect a larger change. This results in a largerrelative phase shift between the counterpropagating light beams and alarger amplitude intensity based signal when the two beams areinterferometrically recombined on the beamsplitter.

In the second mode of operation, the frequency shifter is turned "on"and both counterpropagating light beams are frequency shifted when theypass through it. In this mode of operation the signal generated by anenvironmental disturbance along the sensing loop between thebeamsplitter and the frequency shifter is independent of the position ofthe frequency shifter. Only after they pass through the frequencyshifter do they differ in frequency by the amount of the frequency shiftF in the loop. This is so, even though there are actually two sensingregions between the beamsplitter and the frequency shifter in theclockwise direction and between the beamsplitter and frequency shifterin the counterclockwise direction.

The functional difference between the counterdirectional orientation isthat the phase of the disturbance changes by 180 degrees between theregions. When an environmental effect occurs it changes the effectiveoptical pathlength of a region of the loop. Since one of thecounterpropagating light beams is cycling faster than the other if theoverall pathlength of both light beams increases the faster oscillatinglight beam will increase in phase relative to the more slowlyoscillating counterpropagating light beam. This phenomenon, which isposition independent, provides a measure of the magnitude of theenvironmental effect. By comparing the output of the system with thefrequency shifter switched in the "off" and "on" position dependent andposition independent modes, the location as well as the magnitude of thedisturbance may be deduced.

It is also possible to avoid the inconvenience of switching betweenoperating modes by devising multiple Sagnac loop systems where a centralloop is common to all the Sagnac light loops and performs multisensingcapability. Because a very short coherence length light source may beused in this system, crosstalk between the Sagnac sensing loops iseliminated. One of the loops operates continuously in the mode whereboth counterpropagating beams are at the same frequency to determineposition while the second loop operates with the light beams atdiffering frequency to measure the magnitude of the disturbance. Both ofthese loops have a common region consisting of a single fiber that isused to measure the magnitude and location of an environmentaldisturbance.

For certain applications it is desirable to measure more than one typeof environmental effect, such as strain, temperature and acoustics alongthe same fiber. This can be done by using multiple light sources thatvary in their output frequency to form in effect multiple equations inmultiple unknowns that may be solved simultaneously to extract thevarious environmental disturbances. Further separation can beaccomplished by using electronic filtering to separate low and highfrequency environmental signals. These embodiments may be implemented byusing multiple sets of loops operating at different wavelengths. Becausethe multiple Sagnac interferometric techniques described here supportthe usage of very low coherence length light sources many such loops maybe supported.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified schematic of the distributed Sagnac sensor of thepresent invention;

FIG. 2 is a schematic of the frequency shifting switch of thedistributed Sagnac sensor as shown in FIG. 1;

FIG. 3 is a schematic of a mechanical frequency shifting switchutilizable with the distributed Sagnac sensor as shown in FIG. 1;

FIG. 4 is a graphical illustration of a temperature type changingenvironmental effect plotted against the travel time of the twocounterpropagating light beams in the device of FIG. 1;

FIG. 5 is a graph illustrating the position dependent response due to atime changing environmental effect which induced a fraction of a fullwave of relative pathlength difference between the counterpropagatinglight beams;

FIG. 6 is a detailed schematic of a dual mode Sagnac distributed sensorusing both facets of a light source;

FIG. 7 is a detailed schematic of a dual mode Sagnac distributed sensorusing a single facet of a light source; and,

FIG. 8 is a detailed schematic of a dual mode Sagnac distributed sensorproviding optical isolation between operational modes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown the basic Sagnac distributed sensorof the present invention. The Sagnac distributed sensor of the presentinvention has a light source 11 which may be a light emitting diode(LED) or pigtailed superradiant diode. Light source 11 is opticallyaligned with a beamsplitter 13. One output of the beamsplitter 13 isconnected to an output photo detector 15.

Another output of beamsplitter 13 is connected to a phase modulator 17.Another output of beamsplitter 13 is connected to a frequency shifterswitch 19. Both phase modulator 17 and frequency shifter switch 19 areconnected to a sensing optical fiber 21. Optical fiber 21 provides theoptical path means for conduction of light from light source 11.Frequency shifter switch 19 may be of the acousto optical type. A pairof breaks in the sensing optical fiber 21 are shown to emphasize therelatively longer length of optical fiber 21 with respect to the otheroptical connections shown in FIG. 1.

An oscillator 23 is connected to frequency shifter switch 19 via anamplifier 24. Output photodetector 15 is electrically connected to asynchronous demodulator 25. Synchronous demodulator 25 is electricallyconnected to an oscillator 27. Oscillator 27 is also connected to phasemodulator 17. A controller 29 is connected to both synchronousdemodulator 25 and oscillator 23.

For optimum workability, a fiber optic sensor will contain several otherconditioning devices which are omitted from FIG. 1 in order to clearlyillustrate the working principles contained therein. These types ofdevices are clearly taught in the literature on fiber optic rotationsensors. Light from light source 11 propagates through beamsplitter 13.The light then counter propagates about the optical fiber 21. One beamof light propagates in the clockwise direction passing first throughphase modulator 17, and then through frequency shifter switch 19 beforereturning to beamsplitter 13. Another beam of light propagates in thecounterclockwise direction passing first through frequency shifterswitch 19, and then through phase modulator 17 before returning to beamsplitter 13. This arrangement insures that the two counter propagatinglight beams arrive at detector 15 at the same frequency.

Frequency shifter switch 19 is activated to shift the frequencies of thecounterpropagating beams of light. Demonstrations using a frequency of80 MHz has been found to work satisfactorally in an acoustoopticalfrequency shifter. By virtue of its position at the end of one side ofthe loop, frequency shifter switch 19 shifts the frequency of one of thecounter propagating beams of light before it enters optical fiber 21 andshifts the frequency of the other counterpropagating beam only after ithas already passed through optical fiber 21. The environmental effect ordisturbance causes an optical pathlength change due to contributionsfrom (1) change in refractive index, (2) thermal elongation of theoptical fiber 21, and (3) waveguide shifts due to strain. These effectsare complex, and under a given set of circumstances can interact indifferent ways.

An environmental effect occurring along sensing optical fiber 21 will beexperienced by both of the counterpropagating beams. The clockwisepropagating beam leaves the beam splitter 13 and passes through phasemodulator 17 and frequency shifter switch 19 where its frequency isshifted. The counterclockwise propagating beam passes by frequencyshifter switch 19 where its frequency is also shifted and then throughthe midpoint of elongate optical fiber 21 and then through phasemodulator 17. The counterpropagating light beams then recombine atbeamsplitter 13 at the same frequency so that their phases can becompared. Since the two light beams propagated through optical fiber 21at a different frequency, if the loop is disturbed causing an opticalpathlength change in the sensing fiber 21, the two beams will experiencea relative phase shift. This effect is termed a "nonreciprocal" phaseshift.

As an example, temperature increases will cause optical fiber 21 toexperience an optical pathlength change. This will cause the phases ofthe counterpropagating light beams to shift with respect to each other.The magnitude of the temperature change will determine the extent of thephase shift. The same is true for other environmental disturbances suchas strain and tension.

In a second mode of operation, the frequency shifter switch 19 isdeactivated, resulting in both counterpropagating light beams being atthe same frequency. Under this condition, the environmentally inducedphase differences are transmitted through to the detector 15. This modeof operation allows detection of a momentary phase shift dependent uponthe location of the environmental disturbance along optical fiber 21. Asan example, suppose that an optical disturbance occurs along the tophalf of the optical fiber 21 shown in FIG. 1.

For ease of illustration, the clockwise propagating light beam will bereferred to as the first beam, and the counterclockwise beam will bereferred to as the second beam. The first beam experiences theenvironmental disturbance upon the upper half of optic fiber 21 beforeit reaches the midpoint of the optic fiber and experiences a phase shiftin relation to the severity of the environmental disturbance. The secondbeam will have experienced the effect after having passed through thelower half of the optical fiber 21, after traversing the midpoint, andjust before returning to the detector. Because the disturbance ischanging with time and the first and second beams arrive at differenttimes, they will see different optical pathlengths and their relativephase will change. When the two beams recombine interferometrically onthe beamsplitter, their phase difference results in an intensitymodulated light signal that falls onto the detector 15. The signal fromdetector 15 is detected within synchronous demodulator 25. Synchronousdemodulator 25, as well as phase modulator 17, is driven by oscillator27. The demodulated signal, a synchronous demodulator 25, is convertedto a current or voltage output. This output goes to a controller 29which switches the oscillator 23 driving the frequency shifter switch 19on and off between the two operating modes. The outputs from thecontroller 29 indicative of the location and amplitude of theenvironmental signal correspond to the outputs from the synchronousdemodulator 25 during each mode of operation. Frequency shifter 19 isdriven by a signal from oscillator 23 which is amplified in amplifier24.

The phase shift just described for an environmental disturbance occursover a very small time frame. The extent of the phase shift of thesecond beam depends upon the severity and time dependence of theenvironmental disturbance, and the extent of the phase shift of thefirst beam in catching up with the phase shift of the second beam isalso dependent upon the severity and time dependence of theenvironmental disturbance, and to the same extent as that occurring inthe second beam. The difference between the time of arrival of thesecond beam and the time of arrival of the first beam, however, isdetermined by the location of the environmental disturbance along theoptic fiber 21 of the disturbance.

In the case of an environmental effect occurring on the upper portion ofthe loop very near the end, it is clear that the clockwise beam wouldexperience the effect very early during its travel through opticalfiberbeam 17, while the counterclockwise would experience thedisturbance very late in its travel through optical fiber 17. This wouldrepresent the case for the maximum time difference between the arrivalof the first and second beams at the environmental disturbance location.This will result in the maximum signal provided the induced phase shiftbetween the beams is less than one wave over the time interval betweenthe first and second beams arrival. This latter condition is usually thecase for many applications.

However, if the environmental disturbance occurred close to the midpointalong the upper portion of optical fiber 21, the clockwise beam wouldexperience the disturbance very late during its travel through opticalfiber 17, while the counterclockwise beam would experience thedisturbance just after passing the midpoint and very early in its travelthrough the top portion of optical fiber 17. This would represent thecase for minimum duration between the beginning of the phase shift ofthe second beam at the detector and the beginning of the phase shift ofthe first beam; i.e., the time of arrival difference will approach zero.

In this case the phase shift of the second beam would be immediatelyfollowed by a phase shift in the first beam. The phase of the first beambegins "catching up" with the second before the phase of the second beamhas time to shift very far away from the phase of first beam. ; Thus therelative phase shift between the first and second beams is of relativelyshort duration.

Thus, in the second mode of operation, the relative phase differencebetween the first and second beam is dependent both upon the degree ofoptical pathlength change, which translates into a relative phase shift,and is also dependent upon the time of arrival interval between thefirst beam and the second beam which depends on the position of thedisturbance in the loop.

The relative phase shift has contributions due to the severity andlocation of the environmental disturbance. Thus the relative phaseshifts carry composite information about the severity and location of anenvironmental disturbance. The composite information, without more,would be ineffective in helping identify separately the severity andlocation of the environmental disturbance. However, since we can operatein frequency shift mode as mentioned above, we can ascertain themagnitude of the environmental disturbance upon the whole of the opticalfiber 21. Also known is the relationship between the magnitude of theenvironmental disturbance along the length of optical fiber 21 and therelative phase shift produced due to such disturbance. Indeed, themagnitude of the disturbance is measured according to the phase shift soproduced. Once the phase shift due to the magnitude of the environmentaldisturbance along the length of optical fiber 21 is ascertained, it canbe used to measure the location in combination with the signal derivedfrom the two beams counterpropagating at the same frequency.

The device of FIG. 1 is capable of operating in frequency shift mode todetermine the magnitude of the disturbance along optical fiber 21 or iscapable of operation with the frequency shifter switch 19 de-energizedto obtain the composite disturbance severity/location information. Thedevice of FIG. 1, however, must be switched between these two operatingmodes. Another mode of operation would include operation with thefrequency shifter in the de-energized state. Once a disturbance occursthe relative phase shift information is obtained. The device can then beswitched to the frequency shift mode to determine the magnitude of thedisturbance. Once this composite information is determined, the locationinformation is readily calculated by ratioing the phase shift due to themagnitude of the disturbance to the position dependent phase shift.

There are a number of ways to implement frequency shifting switch 19.FIGS. 2 and 3 illustrate two methods based upon the use of acoustoopticmodulators. FIG. 2 illustrates frequency shifter switch 19 connectedbetween the ends of optical fiber 21. Beginning at the left, opticalfiber 21 is in optically connected to a lens 53. Lens 53 is in opticalalignment with an acoustooptic modulator 57. Acoustooptic modulator 57can be made from a crystal of TeO₂ and have a LiNbO₃ transducer toacoustically activate the crystal. Acoustooptic modulator 57 forms adirect optical alignment with a lens 63. In addition acoustoopticmodulator 57 forms an angled optical alignment with a lens 65. Lens 63is connected to a short length of optical fiber 69. Lens 65 is connectedto a short length of optical fiber 67. Both short lengths of opticalfiber 67 and 69 are connected to a fiber beamsplitter 71. Fiber beamsplitter is connected to optical fiber 21, also shown on FIG. 1.

Referring to FIG. 2, the first method of operation entails a light beampropagating through optical fiber 21 and then collimated by graded indexlens 53. The resulting light beam 55 then enters the acoustoopticmodulator 57 at approximately the Bragg angle for near optimum frequencyshifting conditions. When the power to the acoustooptic modulator 57 isswitched off by the controller 29 of FIG. 1, the light passes throughthe modulator 57 without being without being frequency shifted. Thenon-frequency shifted light is then collected by a lens 53 and coupledinto a short length of optical fiber 69. The light beam then passesthrough a fiber beamsplitter 71 before again being coupled into theoptical fiber 21 of the Sagnac distributed sensor of FIG. 1.

When the power to acoustooptic modulator 57 is switched to the oncondition, the majority of the light beam is frequency shifted andangularly directed away from the non frequency shifted light beam. Thislight beam 59 is collected by the lens 65 and coupled into the opticalfiber 67 before passing through the fiber beam splitter 71 before beingagain coupled into the optical fiber 21 of the Sagnac distributed sensorof FIG. 1.

Referring to FIG. 3, the frequency shifting switch 19 is as shown inFIG. 2 up to and including the acoustooptic modulator 57. In FIG. 3,however, there is only one lens 63, and it is mounted upon a mechanicalactuator 75. Lens 63 is optically coupled directly into optical fiber 21of FIG. 1. In the second method of operation, light again enters theoptical fiber 21 and again is collimated by graded index lens 53. Theresulting light beam 55 then enters the acoustooptic modulator 57, asbefore, at approximately the Bragg angle for near optimum frequencyshifting conditions. The output light beams, 59 and 61, however, arecollected by a single lens 63 moved by a mechanical actuator 75.Mechanical actuator 75 is moved to the position which allows optimumcoupling of the light beams 59 and 61 into the optical fiber 21. Whenthe power to the acoustooptic modulator 57 is switched off by thecontroller 29 of FIG. 1, the light passes through the modulator 57without being frequency shifted. The unfrequency shifted light is thencollected by the lens 63 when the mechanical actuator 75 moves lens 63into optical alignment with light beam 61. When the power to theacoustooptic modulator 57 is switched on by the controller 29 of FIG. 1,the light passes through the modulator 57 and is frequency shifted. Thefrequency shifted light beam 59 leaves modulator 57 and is collected bylens 63 after lens 63 is moved into position by mechanical actuator 75.The light beam then passes through into optical fiber 21 of the Sagnacdistributed sensor of FIG. 1.

Referring to FIG. 4 in conjunction with FIG. 1, a graph illustrating theworkings of the theory is shown. The ordinate of the graph representsthe temperature at any point along the optical fiber 21. The abscissarepresents the time delay caused by the differing time of arrival foreach of the counterpropagating beams. Assuming a sudden drop intemperature along optical fiber 21, the section of fiber experiencingthe temperature drop was originally at temperature T_(a) and isproceeding to temperature T_(b). The curve represents the temperaturechange of the optical fiber 21 as a function of time.

To better visualize the theory of the invention, consider a "pulse" oflight emitted from light source 11 at time equal to zero, T₀. Assumethat the disturbance is on the upper part of the optical fiber 21. Thetemperature of the fiber is made to begin dropping as soon as the"pulse38 of light leaves the light source 11. The light passes throughbeam splitter 13 and is split into counterpropagating light beams. Asthe light beams propagate about optical fiber 21, the temperature at thepoint in question along optical fiber 21 continues to drop. As theclockwise quantity of light passes the section of optical fiber 21experiencing the temperature drop, it is at temperature T₁. Theclockwise quantity of light experiences a phase delay dependent upon therefractive index of the temperature affected portion of optical fiber21, to the extent of its temperature at the time the clockwise quantityof light passes it. After the clockwise pulse of light experiences thephase delay, it continues in a clockwise direction.

Meanwhile, the counterclockwise beam of light is approximately oppositethe clockwise beam of light, but on the underside of the loop. As thecounterclockwise beam continues around the loop, the clockwise beamalready having experienced the temperature T₁, the temperature of thedisturbance region of optical fiber 21 continues to drop. By the timethe counterclockwise beam of light reaches the disturbance region, thetemperature of the region has dropped to T₂. Since T₂ is lower than T₁,the optical fiber contracts, and the optical pathlength is shorter, andthe phase of the counterclockwise pulse is delayed relative to theclockwise light beam. A more severe phase shift translates into agreater delay time. After being phase shifted, the counterclockwise beamof light continues on to beamsplitter 13 and then to detector 15, therelative phase change results.

As previously discussed, the information pertaining to the magnitude ofthe disturbance, determined using the frequency shift mode, as describedabove, can be used to determine the position along optical fiber 21 atwhich the environmental effect occurred. The data relating informationpertaining to the severity of the environmental disturbance along theloop to the composite information obtained due to the momentary phaseshift is readily integratable into a normalization chart. FIG. 5 showsthe curve for such a normalization. The normalized values of theordinate of FIG. 5 range from +1 to -1. A positive value represents apositive phase shift, corresponding to the clockwise counterpropagatinglight beam being shifted in the positive direction with respect to theother beam. A negative value represents a negative phase shift,corresponding the clockwise counterpropagating light beam being shiftedin the negative direction with respect to the other beam. The abscissaof the graph of FIG. 5 represents the distance around the circumferenceof the optical fiber 21.

The center of the graph corresponds to a disturbance occurring in thecenter of the optical fiber 21. Even though a disturbance occurring atthe center of the optical fiber 21 produces no relative phase shift, themagnitude of the disturbance information with which the curve of FIG. 5is normalized will nevertheless reveal the location of the disturbancealong optical fiber 21. The attenuation information illustrated in thegraph of FIG. 5 can easily be converted to tabular form for use with adigital computer.

Referring to FIG. 6, a detailed schematic of a Sagnac distributed sensorwhich simultaneously determines the magnitude and location of anenvironmental disturbance is shown. The necessity to switch to and fromthe frequency shift mode is eliminated. A common optical fiber path isused for simultaneous measurement of the position and severity of anenvironmental disturbance. Light from a light source 101, which may be alight emitting or superradiant diode, is coupled out of both ends 103aand 103b of light source 101. Light coupled into end 103a propagatespast a fiber coupler 105 and a polarizing element 107 before being splitinto two counterpropagating light beams by a beamsplitter 109. Lightpolarization control may be handled by using polarization preservingoptical fiber or polarization scrambling techniques described in theliterature. A clockwise beam 111 and a counterclockwise beam 113 arecirculated through an optical fiber 115. The clockwise beam 111 firstpasses through an optical phase modulator 117 which is driven at afrequency w by an electrical oscillator 119. The resulting phasemodulation between the counterpropagating light beams 111 and 113 isused for detection of a disturbance as described above.

The two counterpropagating light beams 111 and 113 complete their pathsabout optical fiber 115, recombine on the beamsplitter 109, and aredirected through the polarizer 107 and fiber coupler 105 to an outputdetector 121. Output detector 121 is electrically connected to asynchronous demodulator 123. Electrical oscillator 119 is also connectedto synchronous demodulator 123. Synchronous demodulator 123 is alsoconnected to a normalization circuit 125. Synchronous demodulator 123 isused to convert the output of output detector 121, produced due to theenvironmental effect acting upon optical fiber 115, into a voltageoutput. Output detector 121, and synchronous demodulator 123 measure themomentary relative phase shift of the counterpropagating beams 111 and113. The location and severity of the environmental disturbance on theoptical fiber 115 will determine the amplitude of this voltage output.The output from synchronous demodulator 123 is electrically sent tonormalization circuit 125 for processing.

At end 103b of light source 101, light is directed to a coupler 127 anda polarizer 129 before being split by a beamsplitter 131a into aclockwise counterpropagating light beam 133 and a counterclockwisecounterpropagating light beam 135. Beams 133 and 135 also propagatealong the sensing portion cf optical fiber 115. This is accomplished byclockwise propagation of beam 133 from beamsplitter 131a, throughbeamsplitter 109, phase modulator 117, the length of optical fiber 115,a fiber coupler 132, a phase modulator 137, and a frequency shifter 139.Counterclockwise propagation of beam 135 follows the same path, but inreverse order.

Phase modulator 137 is electrically connected to an oscillator 141. Thephase modulator 137 is driven at a frequency w₂ by oscillator 141. Thefrequency w₂ is chosen to facilitate its demodulation from frequency w.Counterpropagating light beams 133 and 135 recombine on the beamsplitter131a and are directed through polarizer 129 and coupler 127 to adetector 143. The relative phase of the two light beams 133 and 135 oncethey have returned will depend upon the bulk length of the optical fiber115. However, because of the action of the frequency shifter 139 inoffsetting the total path lengths of the counterpropagating light beams133 and 135 through the optical fiber 115, the magnitude of theenvironmental effect along the whole optical fiber 115 is measured asproportional to the signal sent to frequency shifter 139 to keep therelative phase difference of the light beams 133 and 135 at zero. Thisprocess is known as phase nulling.

A synchronous demodulator 145, electrically connected to detector 143,is used demodulate the signal received at detector 143. Synchronousdemodulator 145 is set to demodulate at frequency w2. Synchronousdemodulator 145 outputs a voltage electrically supplied to an integrator147 and a voltage controlled oscillator 149. Voltage controlledoscillator 149 is electrically connected to frequency shifter 139.Voltage controlled oscillator 149 supplies a signal which readjusts thefrequency of frequency shifter 139 so that the relative phases of thelight beams 133 and 135 are counterbalanced and the system is nulledout. The amplitude of the signal controlling frequency shifter 143 isindependent of the position of the disturbance along the length ofoptical fiber 115. Detector 143, synchronous demodulator 145 andintegrator 147 measure the steady state relative phase shift betweenbeams 133 and 135.

The output voltage from integrator 147 is also supplied to normalizationcircuit 125. Normalization circuit 125, which also receives output fromsynchronous demodulator 123, can now produce two output signals, oneproportional to the severity of the disturbance, and the otherproportional to the location of the disturbance along optical fiber 115.

FIG. 7 illustrates another embodiment of the present invention. A singlefaceted light source 171 supplies light to a beamsplitter 173. The lightis split into a light beam 175 and a light beam 177. The remainder ofthe system operation is identical to that previously discussed for FIG.6. Light beam 175 acts in the same manner as light which propagates fromend 103a of light source 101 of FIG. 6. Light beam 177 acts in the samemanner as light which propagates from end 103b of light source 101 ofFIG. 6.

FIG. 8 illustrates another embodiment of the present invention. Thesingle faceted light source 171, beamsplitter 173 and light beams 175and 177 are present as was previously shown in FIG. 7. Here, a beamssplitter 191 is provided to split light beam 175. A fiber coupler 193 isprovided to couple light beam 133 into the optical fiber 115.Beamsplitter 191 splits light beam 175 into a clockwise light beam 195and a counterclockwise light beam 197. Light beam 133 and light beam 195combine at fiber coupler 193 to form the clockwise propagating lightbeam 111. Likewise, light beam 197 and light beam 135 are combined atfiber coupler 132 to form counterclockwise propagating light beam 113.This configuration provides isolation between the two sensing circuitsof the dual mode sagnac sensor. The counterpropagating light beams 195and 197 which interfere and provide an output signal on output detector121 will not interfere with the counterpropagating light beams 133 and135 which interfere to produce an output signal on output detector 143.This is due to using a short coherence length light source 171 andensuring that the optical pathlength offsets between the two loops arelarge compared to the light source coherence length.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the opticalcircuit elements, the medium through which the light propagates, andlight beam conditioning devices, as well as the details of theillustrated configuration may be made without departing from the spiritand scope of the invention.

I claim:
 1. A frequency shifter switch comprising:transduction means foradmitting light and acoustically shifting the frequency of said admittedlight wherein said transduction means further comprises:a TeO₂ crystal;and, a LiNbO₃ piezoelectric transducer attached to said TeO₂ crystal;and, optical coupling means for receiving and transmitting bothfrequency shifted and non-frequency shifted light.
 2. A frequencyshifter switch comprising:transduction means for admitting light andacoustically shifting the frequency of said admitted light; and, opticalcoupling means for receiving and transmitting both frequency shifted andnon-frequency shifted light wherein said optical coupling means furthercomprises:a first lens adjacent said transduction means, and generallylinearly oriented to receive said non-frequency shifted admitted light;a second lens adjacent said transduction means, and generally linearlyoriented to receive said frequency shifted admitted light; and an opticcoupler having a first input optically connected to said first lens, asecond input optically connected to said second lens, and at least oneoutput for transmitting light from said first and said second lenses. 3.The frequency shifter switch of claim 2 wherein said transduction meansfurther comprises:a TeO₂ crystal; and. a LiNbO₃ piezoelectric transducerattached to said TeO₂ crystal.
 4. A frequency shifter switchcomprising:transduction means for admitting light and acousticallyshifting the frequency of said admitted light; and optical couplingmeans for receiving and transmitting both frequency shifted andnon-frequency shifted light wherein said optical coupling means furthercomprises:a lens adjacent said transduction means, and generallylinearly oriented to receive light passing through said transductionmeans; a mechanical actuator, movably supporting said lens such thatwhen said transducer produces non-frequency shifted light, saidmechanical actuator means said lens in a position to capture saidnon-frequency shifted light, and when said transducer produces frequencyshifted light, said mechanical actuator moves said lens in a position tocapture said frequency shifted light; and, an optic fiber opticallyconnected to said lens.
 5. The frequency shifter switch of claim 4wherein said transduction means further comprises:a TeO₂ crystal; and, aLiNbO₃ piezoelectric transducer attached to said TeO₂ crystal.
 6. Afrequency shifter switch comprising:a first lens adjacent saidtransduction means, and generally linearly oriented to transmit light;transduction means for admitting said transmitted light and acousticallyshifting the frequency of said admitted light; a second lens adjacentsaid transduction means, and generally linearly oriented to receive saidnon-frequency shifted admitted light; a third lens adjacent saidtransduction means, and generally linearly oriented to receive saidfrequency shifted admitted light; and, an optic coupler having a firstinput optically connected to said second lens, a second input opticallyconnected to said third lens, and at least one output for transmittinglight from said first and said second lenses.